Changing the concentration of the reactants
4 marks · NormalThe table below gives the concentration of a reactant and the corresponding average rate of reaction measured at the start of an experiment.
| Concentration of HCl (mol/dm³) | Average rate (g/s) |
|---|---|
| 0.50 | 0.004 |
| 1.00 | 0.008 |
| 1.50 | 0.012 |
| 2.00 | 0.016 |
Describe the pattern shown by the data.
Predict the average rate if the concentration is increased to 2.50 mol/dm³.
Explain your answer to part b.
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(a) As concentration doubles, rate doubles / directly proportional relationship
(b) 0.020 g/s
(c) The rate is directly proportional to concentration, so increasing concentration by a factor increases rate by the same factor.
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Marking points
B1 (a) · 1 mark · Correctly describes pattern (doubling concentration doubles rate/relationship is directly proportional).
A1 (b) · 1 mark · Predicts correct average rate for 2.50 mol/dm³: 0.020 g/s (allow calculator values as correct). (follow-through)
E1 (c) · 1 mark · States/directly refers to the relationship between concentration and rate (direct proportionality). (follow-through)
E2 (c) · 1 mark · Uses data or multiplication factor to justify answer. (follow-through)
Worked solution
(a) The rate increases proportionally with concentration; doubling concentration doubles the rate (directly proportional). (b) At 2.00 mol/dm³ rate is 0.016 g/s. At 2.50 mol/dm³ (which is 1.25 times 2.00), the rate will be 1.25 × 0.016 = 0.020 g/s. (c) Because the data show that rate is directly proportional to concentration, so increasing the concentration by 1.25 times increases rate by 1.25 times.